This invention relates to threaded joints for tubulars and, more particularly, to an improved thread design which has particular advantages when used with pipe joints constructed from composite material such as fiberglass reinforced plastic (FRP). The invention is particularly adapted for use in a pin and box threaded joint type tubulars.
It is important and a great advantage that all equipment and tubulars used in and around oil wells be sufficiently durable to withstand rough handling and abuse. Tubulars should be capable of functioning in a dirty environment. Whether the tubulars are suspended in the well or used above ground, the tubulars should be rugged and resist damage. The nature of the materials being handled by oil field tubulars makes it important that the tubulars provide a reliable seal and do so under a variety of conditions and when exposed to rather high pressures. The tubulars should be designed such that during makeup (threading of adjacent sections together), the tubulars will not readily cross-thread because of misalignment.
Composite pipe joints should be designed so that when the joints are made up and torqued, damaging strains induced into the box and pin should be minimized to prevent laminate damage and cracking and damage to the threads in the pipe sections. In addition, the sections should be reusable, in that, a low breakout torque be required and thread galling be reduced to an acceptable level. In addition, the pipe should be designed so that it can be quickly made up and broken down to reduce the installation costs.
In the past, pipe joints constructed from metallic materials have had threads designed to correspond to the characteristics of these materials. However, thread designs for metallic tubulars function differently in fiberglass and can present significant disadvantageous when used in a fiberglass pipe joint.
In the past, fiberglass tubulars have been commonly joined with threaded connections. One of the standard connections is what is known in the industry as a "8RD EUE" threaded connection. Although these threaded joints have functioned for their intended purpose, they have not proved entirely satisfactory, in that, they have a tendency to cross-thread which can effectively destroy the threads in a joint. If the threading operation is slowed down to a rate where the possibility of cross-threading is substantially reduced, higher installation costs are experienced in the field. In addition, 8RD EUE threads induce strain in the box and pin due to makeup torque causing failures in the joint in some cases.